linux-kernel - [PATCH v3] mm/filemap: don't decrease mmap

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Date: Tue, 26 Mar 2024 14:50:26 +0800
From: Liu Shixin <liushixin2@...wei.com>
To: Andrew Morton <akpm@...ux-foundation.org>
CC: Jan Kara <jack@...e.cz>, Matthew Wilcox <willy@...radead.org>, Alexander
 Viro <viro@...iv.linux.org.uk>, Christian Brauner <brauner@...nel.org>,
	<linux-fsdevel@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
	<linux-mm@...ck.org>, Liu Shixin <liushixin2@...wei.com>
Subject: [PATCH v3] mm/filemap: don't decrease mmap_miss when folio has workingset flag

If there are too many folios that are recently evicted in a file, then
they will probably continue to be evicted. In such situation, there is
no positive effect to read-ahead this file since it is only a waste of IO.

The mmap_miss is increased in do_sync_mmap_readahead() and decreased in
both do_async_mmap_readahead() and filemap_map_pages(). In order to skip
read-ahead in above scenario, the mmap_miss have to increased exceed
MMAP_LOTSAMISS. This can be done by stop decreased mmap_miss when folio
has workingset flag. The async path is not to care because in above
scenario, it's hard to run into the async path.

Signed-off-by: Liu Shixin <liushixin2@...wei.com>
Reviewed-by: Jan Kara <jack@...e.cz>
---
v2->v3: Update the title and comment. And add reviewed-by from Jan.

Andrew, please update patch[2] with this new patch, thanks.

 mm/filemap.c | 14 ++++++++++++--
 1 file changed, 12 insertions(+), 2 deletions(-)

diff --git a/mm/filemap.c b/mm/filemap.c
index 8df4797c5287..780aad026b26 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -3439,7 +3439,15 @@ static vm_fault_t filemap_map_folio_range(struct vm_fault *vmf,
 		if (PageHWPoison(page + count))
 			goto skip;
 
-		(*mmap_miss)++;
+		/*
+		 * If there are too many folios that are recently evicted
+		 * in a file, they will probably continue to be evicted.
+		 * In such situation, read-ahead is only a waste of IO.
+		 * Don't decrease mmap_miss in this scenario to make sure
+		 * we can stop read-ahead.
+		 */
+		if (!folio_test_workingset(folio))
+			(*mmap_miss)++;
 
 		/*
 		 * NOTE: If there're PTE markers, we'll leave them to be
@@ -3488,7 +3496,9 @@ static vm_fault_t filemap_map_order0_folio(struct vm_fault *vmf,
 	if (PageHWPoison(page))
 		return ret;
 
-	(*mmap_miss)++;
+	/* See comment of filemap_map_folio_range() */
+	if (!folio_test_workingset(folio))
+		(*mmap_miss)++;
 
 	/*
 	 * NOTE: If there're PTE markers, we'll leave them to be
-- 
2.25.1